Apple claims over 1,000 new APIs are included for developers, allowing for a host of new features in your iPhone apps. In-app purchasing will allow for devs to create and vend new content (think new level packs or maps, new comics, special weapons/characters in games, etc). This is a huge new market for iPhone developers who have been frustrated with the initial push in income from an iPhone app which quickly dwindles to a trickle - and a good way to keep users happy with their apps. Dynamic content makes for an app you'd like to come back to more often.

Peer to peer connectivity is now directly accessible from the SDK, meaning you can play your friend via Bluetooth (no pairing required) in games, or transfer files directly to each other through apps that could come out - or updates to existing apps.

Push notification finally going live means quasi-backgrounding. Apps can now send an SMS, trigger an alert, or add a badge even when your app is not running.

The External Accessory framework is pretty cool too - make the iPhone into an external remote for any hardware device, connecting to it through the 30 pin connector or with Bluetooth. Hey Sony - you hear that? My PS3 wants my iPhone to control DVD playback. A $1.99 app is way cheaper than that $30 DVD remote you sell.

There's a ton more - like Maps included in-app, or access to the iPod library including playback etc. Download the SDK and start writing!